Magnetic resonance tomography systems are imaging devices that, in order to depict an examination object, align nuclear spins of the examination object with a strong outer magnetic field and by way of a magnetic alternating field excite the same for precession about this alignment. The precession or return of the spins from this excited state into a state of less energy in turn generates a response in the form of a magnetic alternating field that is received by antennas.
With the aid of magnetic gradient fields, spatial encoding is impressed on the signals and this subsequently permits assignment of the received signal to a volume element. The received signal is then evaluated, and a three-dimensional imaging representation of the examination object is provided. The signal may be received using local antennas, so-called local coils, arranged directly on the examination object to achieve a better signal-noise ratio.
The resonance frequency of the nuclear spins, also called the Larmor frequency, is directly proportional to an outer static or quasi-static magnetic field including the static magnetic field B0 and the gradient fields. A magnetic resonance scan is possible in a region in which the static B0 magnetic field is sufficiently homogeneous. This region may be limited to a sphere with a diameter of a few tens of centimeters. To examine larger body regions, it is necessary to move these through the homogeneous magnetic field region, for example, on a patient bed.
It is, for example, known to provide patient beds with a cable pull that is connected to an encoder and supplies a signal as a function of the position of the patient bed. Such cable pulls may be associated with play, for example, on a change of direction. In addition, it is first of all necessary to establish a relationship between the encoder's position information and the magnetic field, which may possibly also change from examination to examination.